1. Field of the Invention
The present invention relates to a magnetic element such as an inductor used in electric equipment and a method of manufacturing the magnetic element.
2. Description of the Related Art
In recent years, further improvement in performance of magnetic elements such as an inductor is demanded. Together with this improvement in performance, downsizing of magnetic elements is also requested, so that the size of the magnetic elements cannot be made larger for the purpose of improving performance. On the other hand, currently available magnetic elements include a drum type, a lamination type, and the like.
A schematic structure of a magnetic element of drum type is shown in FIG. 20. In the magnetic element of drum type, an air gap 103 exists between an upper flange portion 101 and a lower flange portion 102 of a drum type core 100 included in the magnetic element, and the existence of the air gap secures extension (which means not to decrease) of an L value (inductance) in a direct current superposition. However, when the air gap 103 exists, there is a problem of magnetic flux leakage to the outside. Also, when the air gap 103 exists, the L value decreases slightly.
Further, in the magnetic element of drum type, if downsizing (thinning) is advanced, the upper flange portion 101 and the lower flange portion 102 constituting the drum type core 100 become thin. Accordingly, when stress is applied to the upper flange portion 101 and the lower flange portion 102, the risk of breakage increased. In other words, there is a certain degree of limitation in downsizing of the magnetic element of drum type. Further, in addition to the problem of breakage, when downsizing of the magnetic element of drum type advances, it becomes difficult to reduce resistance to an electric current as compared to a magnetic element of large size, so that a large current cannot flow. Furthermore, it is demanded that decrease of an inductance (L value) in direct current superposition in a magnetic element is low, and also it is demanded that a loss in a high frequency region is small.
Incidentally, as a technique to obtain a large L value in the above-described magnetic element of drum type, it is conceivable to arrange a material having high magnetic permeability (ferrite for example) at the position of the air gap. However, when a material having high magnetic permeability such as ferrite is arranged, magnetically saturation can easily occur, and inversely, the magnetic permeability decreases at a predetermined current value or larger, which finally becomes equal to that of an air-core coil. Thus, the magnetic permeability of a material to be arranged should be suppressed to a certain degree. Further, in order to obtain a large L value, other factors (cross sectional area of a magnetic path for example) which decide the inductance may be changed. However, such a change leads to enlargement of the magnetic element, so that it goes against the request for downsizing. Consequently, it is difficult to realize a magnetic element that has a large inductance, an excellent direct current superposition characteristic, and a small loss in a high frequency region.
Further, as one type that can be downsized (thinned) among other types of magnetic elements (types of magnetic elements other than the drum type), there is a magnetic element of lamination type. This magnetic element of lamination type is manufactured by laminating in a sheet form, or by using a technique of laminating by printing, and the like. Here, the magnetic element of lamination type is used for a signal of minute electric current, or the like in the current situation. However, the magnetic element of lamination type cannot respond to a large current due to structural limitation, magnetic characteristic limitation, and so on, and in such cases, it cannot function adequately as an inductor.
Specifically, when downsizing is advanced in either of the drum type and the lamination type, generally a characteristic thereof deteriorates. Therefore, improvement in the characteristic is demanded.
Here, as a technique to solve such problems, there is a magnetic element disclosed in Japanese Patent Application Laid-open No. 2001-185421 (refer to Abstract, FIG. 1, FIG. 2, and so on). For the magnetic element disclosed in this patent application, there is adopted a structure such that the L value is increased by eliminating the air gap, and in order to suppress occurrence of magnetic saturation, paste (also referred to as composite; the magnetic member A in the above-described patent document) constituted of metal powder and resin intervenes in a portion of the conventional air gap, and the circumference of a coil is covered by the magnetic member A. Incidentally, when such a structure is adopted, it is found that the magnetic permeability of the magnetic member A constituted of the paste contributes more to the L value and the like than the magnetic permeability of the magnetic member B (ferrite).
In the magnetic member A of the magnetic element disclosed in the above-described patent document, metal powder and resin are mixed in a constant ratio so as to secure fluidity of the paste. Meanwhile, when it is attempted to further improve the magnetic permeability of such a magnetic member A without sacrificing a direct current superposition characteristic, it is conceivable to increase the amount (ratio) of metal powder. However, when the amount of metal powder is increased in the paste, the fluidity of uncured paste is inhibited by that amount. Accordingly, formability thereof deteriorates, and the paste cannot enter a small gap such as a space between windings of a coil, thereby causing a problem that the occurrence of defects increases. Further, since the fluidity of the paste is low, there is also a problem that the production efficiency thereof deteriorates.
Moreover, in a structure having an upper flange portion and a lower flange portion similarly to the magnetic element disclosed in the above-described patent document, the magnetic member A constituted of paste having fluidity flows out while manufacturing. Accordingly, a manufacturing cost thereof is high due to a need of dedicated jig, or the like.